In a high resolution imaging system which is based on orbiting satellites, the quality of the image depends on the systems ability to point to the target and the stability of the satellite platform during the relatively short duration image generation process. The image consists of a swath across the target and must be assembled as a mosaic by the image processing software. As illustrated in FIG. 2, a constellation of satellites orbiting in the same orbital plane can be used to provide a complete scan of the target area. The optical system on each satellite is adjusted by attitude control or other means to scan overlapping swaths. The pointing function requires instantaneous position and attitude data which are, in prior art systems, provided through the use of precision star and earth sensors requiring expensive and complex systems. Because of the relative motion between the target and the imaging satellite, the image generation process is of short duration in the range of between 40 to 60 seconds. During this process it is desirable to hold the aimed optical system steady to prevent smearing, distortion or other image problems. A great deal of effort has been invested to maintain the positional stability of the satellite platform resulting in increased complexity in the associated attitude control system.
An attitude control system of the prior art used for satellite imaging is described in U.S. Pat. No. 4,837,699. In this system, space craft attitude is adjusted by activating axial and radial thrusters in response to an attitude correction signal. The attitude error is sensed by reference to sensors monitoring the relative position of the sun and earth. The use of sun sensors or other celestial means to monitor attitude is complex and costly. The attitude is adjusted to its mission orientation in which the imaging system is pointed at its predetermined target and is maintained in this orientation during orbital flight. During flight the satellite is subject to motions induced by solar torques, on board mechanisms or other sources and the attitude control system must continuously monitor and adjust attitude.
Once the image data is captured, it is transmitted in raw form to a receiving station where it is processed, enhanced and coded for assembly. Complex computer algorithms allow the assembly of the processed images into a mosaic depicting the target area for analysis.
Recent advances in these algorithms allow compensation for pointing inaccuracies and optical platform movement. Such algorithms are described in U.S. Pat. No. 4,688,092.
Image quality is effected by the relative movement between the imaging system and the target. Accordingly the prior art has focused on stabilization through attitude control. Current tolerances for pointing accuracy and movement accommodation are demanding and are a function of the image post-processing as well as the optical payload field of view and resolution. It is the purpose of this invention to adapt the advanced distortion reducing algorithms to allow wider tolerance in pointing accuracy and stability and then to use these increased tolerances to simplify and lower the cost of the system.